Quadrature antenna for magnetic resonance imaging using elliptical coils

ABSTRACT

A quadrature antenna for magnetic resonance imaging systems is provided which includes first and second channels, each of these channels comprising a coil pair. Each of the four coils comprising the antenna is elliptical in shape and lies in a respective plane. The center point of each coil coincides with a reference point on the longitudinal axis of the antenna. The minor axes of the coils of the first channel coil pair are colinear. The minor axes of the coils of the second channel coil pair are also colinear with each other and are perpendicular to the minor axes of the coils of the first channel coil pair. Each of the four planes is inclined approximately forty five degrees (45°) with respect to the longitudinal axis of the antenna.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of magnetic resonanceimaging systems and more specifically to a quadrature antenna for amagnetic resonance imaging system using elliptical coils.

2. Description of the Related Art

Magnetic resonance imaging ("MRI"), also known as nuclear magneticresonance ("NMR") imaging, has become a valuable tool as a safe,non-invasive means for obtaining information in the form of images ofobjects under examination. For example, MRI can be used as a medicaldiagnostic tool by providing images of the whole or selected portions ofthe human body without the use of X-ray photography.

MRI systems take advantage of the magnetic properties of spinning nucleiof chemical species found in the observed object in the followingmanner. Each of the nuclei has an internal spin axis and a magnetic polealigned with the spin axis. The magnetic pole is a vector quantityrepresenting the magnitude and direction of the magnetic field of thenucleus. Application of an external static magnetic field B_(o) causesthe magnetic poles to align themselves along the external magnetic fieldlines.

The MRI system disturbs this alignment by transmitting anelectromagnetic signal to the object. The magnetic field B₁ of thistransmitted electromagnetic signal is circularly polarized and isperpendicular to the static magnetic field B_(o). This signal causes thenuclei to precess about the external static magnetic field lines. Thefrequency of this precession typically is in the radio frequency ("RF")range. More specifically, the precession frequency generally lies withina relatively narrow bandwidth of about 1 to 100 kHz at a centerfrequency of between 1 and 100 MHz.

As the nuclei precess, they radiate an electromagnetic signal having acircularly polarized rotating magnetic field. The frequency of thisrotating magnetic field is gererally equal to the precession frequencyof the nuclei. The radiated signal is received by the MRI system toproduce an image of the object under examination.

The circularly polarized magnetic fields of the transmitted and receivedRF signals described above rotate in a plane perpendicular to the staticmagnetic field B_(o). For convenience, a rectilinear coordinate systemis used here to describe the orientation of these magnetic fields. Thestatic magnetic field B_(o) is assumed to be in the direction of the Zaxis. Therefore, the rotation of the circularly polarized magneticfields is in the X-Y plane.

Quadrature coil antennas have been used in MRI systems to transmit andreceive the RF signals described above. An example of such a quadraturecoil antenna is shown in "Quadrature Detection Coils--A further √2Improvement in Sensitivity," C.N. Chen, D.I. Hoult, and V.J. Sank, J.Maqn. Reson., Vol. 54, 324-327 (1983). This antenna includes acylindrical antenna structure having four saddle coils arranged into afirst coil system and a second coil system. The coils of the first coilsystem are opposite each other. The coils of the second system are alsoopposite each other and are oriented 90° relative to the first coilsystem. Each of the coils is physically 120° wide around the lateraledge of the cylindrical antenna structure and, thus, overlaps each ofits adjacent coils by 30°. With this arrangement, the first coil systemresponds to signals along the X axis while the second coil systemresponds to signals along the Y axis.

The signals received on the first and second coil systems are coherentbut 90° out of phase. A transmit/receive circuit coupled to the antennaadjusts the phase difference between the signals so that they are inphase, and combines these signals to produce a single output signal.Noise in the respective coil systems is assumed to be uncorrelated.Therefore, combination of the signals results in an improvedsignal-to-noise ratio for the MRI system.

A drawback of this quadrature antenna design is the cross-coupling ofthe coil pairs. A voltage in one of the coil pairs induces acorresponding voltage in the other coil pair. This problem isattributable to the extensive overlap of each of the respective coilswith its adjacent coils. It results in correlation of the noise in therespective coil systems or receiving channels of the MRI system andoffsets the gain in sensitivity that is attainable with decoupledchannels.

Another antenna design which has been used in MRI systems is anelliptical coil antenna. For example, a crossed ellipse coil for use inan MRI system is described in "A Crossed Ellipse RF Coil for NMR Imagingof the Head and Neck," T. W. Redpath and R. D. Selbie, Phys. Med. Biol.,Vol. 29, 739-744 (1984). This antenna coil comprises a single conductorwrapped around a cylindrical former for one nearly complete revolutionat an inclination of approximately 45° from the longitudinal axis of theformer. Each end of the conductor is then turned 90° and wrapped aroundthe former, also at an inclination of approximately 45° with respect tothe longitudinal axis of the former. This antenna has a magnetic fieldpolarization axis directed either along or perpendicular to thelongitudinal axis of the former, depending on the specific configurationof the antenna.

The performance of this antenna is limited by its inability to respondto more than one direction, i.e., along both the X and Y axes. It canrespond to the X or the Y component of the circularly polarized magneticfields, but not both simultaneously.

Accordingly, it is an object of the present invention to provide anantenna for an MRI system which has reduced cross-coupling between therespective channels or coil systems of the antenna. It is further anobject of the present invention to provide an antenna for an MRI systemwhich provides signal response of both the X and Y components(perpendicular to the direction of the static magnetic field) of thecircularly polarized magnetic fields of the transmitted and received RFsignals.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and ( combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

To achieve the foregoing objects, and in accordance with the purposes ofthe invention as embodied and broadly described herein, a quadratureantenna for magnetic resonance imaging having a longitudinal axis isprovided which comprises first and second channels. The first channelincludes a first elliptical coil lying in a first plane. The centerpoint of this first coil coincides with a reference point on thelongitudinal axis of the antenna. For convenience, a reference plane isdefined which intersects the longitudinal axis at the reference pointand is normal to the longitudinal axis. The minor axis of the first coillies in this reference plane. The first plane is inclined approximately45° with respect to the longitudinal axis.

The first channel also includes a second elliptical coil lying in asecond plane. The center point of the second coil also coincides withthe reference point. The minor axis of the second coil lies in thereference plane and is colinear with the minor axis of the first coil.The second plane is normal to the first plane and inclined approximately45° with respect to the longitudinal axis.

The first channel also includes means for electrically coupling thefirst channel to an external transmit/receive circuit.

The second channel includes a third elliptical coil lying in a thirdplane. The center point of this coil also coincides with the referencepoint. The minor axis of the third coil lies in the reference plane andis perpendicular to the minor axis of the first coil. The third plane isalso inclined approximately 45° with respect to the longitudinal axis.

The second channel also includes a fourth elliptical coil lying in afourth plane. The center point of the fourth coil coincides with thereference point. The minor axis of the fourth coil also lies in thereference plane and is colinear with the minor axis of the third coil.The fourth plane is rormal to the third plane and is inclinedapproximately 45° with respect to the longitudinal axis.

The second channel also includes means for electrically coupling thesecond channel to an external transmit/receive circuit.

The antenna also preferably includes shielding means for shielding thefirst channel from the second channel.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate a presently preferred embodimentof the invention and, together with the general description given aboveand the detailed description of the preferred embodiment given below,serve to explain the principles of the invention.

FIG. 1A is a perspective view of the first channel of the quadraturecoil antenna of the preferred embodiment according to the invention;

FIG. 1B is a perspective view of the second channel of the quadraturecoil antenna of the preferred embodiment according to the invention;

FIG. 1C is a perspective view of the quadrature coil antenna of thepreferred embodiment obtained by combining the first channel shown inFIG. 1A with the second channel shown in FIG. 1B;

FIG. 2 shows one channel of the quadrature coil antenna of the preferredembodiment of FIG. 1C with radial and longitudinal magnetic fieldpolarization vectors;

FIG. 3A shows a portion of the quadrature coil antenna of the preferredembodiment of FIG. 1C illustrating the connections made in parallel toproduce a radial magnetic field vector;

FIG. 3B shows a portion of the quadrature coil antenna of the preferredembodiment of FIG.lC illustrating the connections made in series toproduce a radial magnetic field vector;

FIG. 3C shows a portion of the quadrature coil antenna of the preferredembodiment of FIG. 1C illustrating the connections made in parallel toproduce an axial magnetic field vector;

FIG. 3D shows a portion of the quadrature coil antenna of the preferredembodiment of FIG. 1C illustrating the connections made in series toproduce an axial magnetic field vector:

FIGS. 4A and 4B illustrate an example of shielding means forelectrostatically shielding the first and second channels of thequadrature coil antenna of the preferred embodiment shown in FIG. 1C;

FIGS. 5A and 5B show two embodiments of a quadrature transmitter circuitfor driving the quadrature coil antenna of the preferred embodiment; and

FIGS. 6A and 6B show two embodiments of a quadrature receiving circuitfor receiving signals from the quadrature coil antenna of the preferredembodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the presently preferredembodiment of the invention as illustrated in the accompanying drawings,wherein like reference characters designate like or corresponding partsthroughout the several drawings.

The quadrature antenna of the preferred embodiment as illustrated inFIG. 1 comprises a first channel A (shown individually in FIG. 1A) and asecond channel B (shown individually in FIG. 1B), each of which in turncomprises a pair of elliptical coils. Preferably, each of the four coilsis wound onto a cylindrical former 100 made of a suitable nonconductingmaterial, such as polyethylene. Former 100 includes a cylinder wall 102,end faces 104, and a longitudinal axis 106. The windings of each of thecoils are incorporated into or on the interior or exterior surface ofcylinder wall 102 of former 100. Preferably, at least one of end faces104 is opened or openable for insertion and removal of an object to beexamined such as the body of a patient. In the preferred embodiment,longitudinal axis 106 is the longitudinal axis of the antenna.

Note that while former 100 of the preferred embodiment comprises acylinder, other geometric shapes are possible. For example, former 100may comprise an elliptic cylinder.

The four coils of the quadrature antenna have a number of commonfeatures. Preferably, each coil is formed on former 100 by winding asuitable elongated conductor, such as wire, on cylinder wall 102. Thenumber of turns in each of the coils is a design choice. There may beonly one turn. However, if a given coil has more than one turn, theturns are positioned so that they are approximately equally spaced aboutcylinder wall 102 of former 100. Preferably, the dimensions of each ofthe four coils is substantially equal to the dimensions of each of theother coils.

Each turn of a given coil lies substantially on a two-dimensional plane.Where only a single turn is used, this turn or winding defines theplane. Where a plurality of turns are used, a central one of the turns,preferably the center turn of the coil, defines the plane.

The plane of each of the coils is inclined approximately 45° withrespect to longitudinal axis 106 of former 100, and the dimensions ofeach of the coils conforms to cylinder wall 102 of former 100. Thus,each of the coils forms an ellipse which represents a planar geometricconic section of cylindrical former 100. As is conventional, the shortaxis of the ellipse is referred to as the minor axis and the long axisis referred to as the major axis. The intersection of the minor andmajor axes or, equivalently, the midpoint between the foci along themajor axis, is the center point of the ellipse through which a coil axisextends perpendicular to the plane of the coil. The center points of thefour coils are coincident at a point at or near the center of theantenna, where the object is located for examination.

As shown individually in FIG. 1A, the quadrature antenna includes afirst channel A. This first channel includes a first elliptical coil 108lying in a first plane 110. The center point of first coil 108 coincideswith a reference point 112 on longitudinal axis 106 of former 100.

A reference plane 114 intersects longitudinal axis 106 at referencepoint 112 and is normal to longitudinal axis 106. This reference planeis not a structural element of the preferred embodiment but is used forconvenience to visualize and understand the three-dimensionalconfiguration of the structural elements of the preferred embodiment.The minor axis of first coil 108 lies in reference plane 114. As noted,first plane 110 is inclined approximately 45° with respect tolongitudinal axis 106.

The first channel also includes a second elliptical coil 116 lying in asecond plane 118. The center point of second coil 116 also coincideswith reference point 112 on longitudinal axis 106 of former 100. Theminor axis of second coil 116 is colinear with the minor axis of firstcoil 108 and, therefore, also lies in reference plane 114. Second plane118 is normal to first plane 110 and is inclined approximately 45° withrespect to longitudinal axis 106.

The first channel also includes means for electrically coupling thefirst channel to an external transmit/receive circuit. The first channelcoupling means of the preferred embodiment, illustrated in FIGS. 1A and1C includes four point connectors 120, 122, 124 and 126 located on theexternal surface of cylinder wall 102 at one end of the minor axes offirst 108 and second 116 coils for ohmically coupling the first channelto the external transmit/receive circuitry. Each of the two ends offirst coil 108 is connected to an opposing one of the four pointconnectors, for example, connectors 120 and 124. Each end of second coil116 is coupled to the first channel coupling means, preferably to one ofthe two remaining point connectors which also oppose one another, forexample, connectors 122 and 126.

The quadrature antenna of the preferred embodiment also includes asecond channel B, as shown individually in FIG. 1B. The second channelincludes a third elliptical coil 128 which lies in a third plane 130.The center point of third coil 128 coincides with reference point 112 onlongitudinal axis 106 of former 100. The minor axis of third coil 128also lies in reference plane 114. Third plane 130 is also inclinedapproximately 45° with respect to longitudinal axis 106.

The second channel B also includes a fourth elliptical coil 132 lying ina fourth plane 134, the center point of which also coincides withreference point 112. The minor axis of fourth coil 132 also lies inreference plane 114 and is colinear with the minor axis of third coil128. The fourth plane 134 is normal to third plane 130 and, as with theother planes, is inclined approximately 45° with respect to longitudinalaxis 106.

The second channel B includes means for electrically coupling the secondchannel to an external transmit/receive circuit, similar to the firstchannel coupling means. The second channel coupling means, asillustrated in FIGS. 1B and 1C, also preferably includes four pointconnectors 136, 138, 140 and 142 located on the external surface ofcylinder wall 102 of former 100 at one end of the minor axes of thirdcoil 128 and fourth coil 132 for ohmically connecting the respectiveends of coil 128 and coil 132 to the external circuitry. Each of theends of third coil 128 is coupled to an opposing one of the pointconnectors, for example, connectors 136 and 140. Similarly, each of theends of fourth coil 132 is connected to the other point connector, inthe example, connectors 138 and 142.

The quadrature antenna of the preferred embodiment comprises thecombination of the first and second channels into a single former 100,as illustrated in FIG. 1C. The respective coils of the first and secondchannels are positioned into or onto former 100 so that their resultantmagnetic field polarization vectors are orthogonal (perpendicular) toone another and each is orthogonal to the static magnetic field vectorB_(o), as described in detail below. Based On the description above, theminor axis of each of the four coils lies in reference plane 114 and thecenter point of each of the coils is coincident at reference point 112.The minor axes of first coil 108 and second coil 116, which arecolinear, are substantially perpendicular to the minor axes of thirdcoil 128 and fourth coil 132, which are also colinear. Each of the coilslies at a 45° angle with respect to longitudinal axis 106. The firstchannel coupling means is preferably approximately 90° (measuring aroundcylindrical wall 102 of former 100) from the second channel couplingmeans. It is to be recognized that this is not a limitation in that theobjects of the invention may be met by locating the first and secondchannel coupling means at other positions or feed points.

It is well known that a current in a coil produces a magnetic fieldinside the coil having an essentially linear magnetic field polarizationvector parallel to the coil axis. The magnitude of the magnetic fieldpolarization vector depends on the number and size of turns in the coiland the magnitude of the current. The direction of the field along thecoil axis is determined by the direction of the current. It is alsoknown that the magnetic fields produced by a plurality of coilssuperpose and result in a net magnetic field polarization vector. Thefields generated by the respective coils may interfere constructively ordestructively. Conversely, a changing magnetic field within a coilinduces a current in the coil, the magnitude of which depends on thestrength and orientation of the magnetic field and the construction ofthe coil. The direction of the current depends on the orientation of thefield relative to the coil.

Each of the coil pairs of the present invention operates in accordancewith these principles. The coils of both coil pairs enclose acylindrical volume, e.g. that of cylindrical former 100 in the preferredembodiment of FIG. 1C, inside which a magnetic field may be transmittedfrom and received by the object under examination. The magnetic fieldpolarization vector of each of the four coils is parallel to the coilaxis of that coil and, thus, is inclined 45° with respect tolongitudinal axis 106 of former 100. The coil pair of each channelcomprises two coils having central axes which are perpendicular to oneanother. As shown in FIG. 2, the linear magnetic field polarizationvectors produced by simultaneously energizing these coils combine toproduce a net magnetic field polarization vector which is eitherperpendicular 144 or parallel 146 to longitudinal axis 106 of former100, depending on the direction of the current in each coil. In thepresent invention, the magnetic field polarization vector of each of thechannels is selected to be perpendicular to the static magnetic fieldvector B_(o), here along the Z axis, since the static field determinesthe alignment of the nuclear poles of the nuclei and, thus, thetwo-dimensional X-Y plane in which precession occurs.

Obtaining the desired direction of the resultant magnetic fieldpolarization vector for a given coil pair, i.e. a given channel, isaccomplished by appropriately configuring the coupling means of thatchannel to couple the respective coils to the external transmit/receivecircuit. Four alternative connection configurations may be used for eachchannel for the four point connector design described above. These fourconfigurations are shown in FIG. 3 for the first channel. Theseconfigurations are equally applicable to the corresponding couplingmeans of the second channel.

A first configuration in which first coil 108 and second coil 116 of thefirst channel are connected in parallel to produce a desired radialmagnetic field polarization vector 144 (perpendicular to longitudinalaxis 106 of former 100 as shown in FIG. 2) is shown in FIG. 3A. Pointconnectors 120 and 126 are electrically coupled as are point connectors122 and 124. A first lead 148 of the transmit/receive circuit iselectrically coupled to point connectors 120 and 126. The point ofcoupling first transmit/receive circuit lead 148 to the coupling meansof one of the antenna channels is referred to here as a firsttransmit/receive terminal. A second lead 150 of the transmit/receivecircuit is electrically coupled to point connectors 122 and 124. Thepoint of coupling second transmit/receive circuit lead 150 to thecoupling means of one of the antenna channels is referred to here as asecond transmit/receive terminal. First coil 108 and second coil 116 arethus coupled in parallel with respect to transmit/receive circuit leads148 and 150. A sinusoidal voltage applied across transmit/receive leads148 and 150 will produce a magnetic field perpendicular to longitudinalaxis 106 as desired (assuming the construction of the first 108 andsecond 116 coils are essentially identical).

A second configuration in which the first coil 108 and second coil 116of the first channel are connected in series to produce a desired radialmagnetic field polarization vector (perpendicular to longitudinal axis106 of former 100 as shown in FIG. 2) is shown in FIG. 3B. Pointconnectors 124 and 126 are electrically coupled. First lead 148 of thetransmit/receive circuit is electrically coupled to point connector 120at the first transmit/receive terminal. Second lead 150 of thetransmit/receive circuit is electrically coupled to point connector 122at the second transmit/receive terminal. First coil 108 and second coil116 are thus coupled in series with respect to transmit/receive circuitleads 148 and 150. A sinusoidal voltage applied across transmit/receiveleads 148 and 150 will produce a magnetic field perpendicular tolongitudinal axis 106 as desired (assuming the construction of first 108and second 116 coils are essentially identical).

Similarly, a third configuration, illustrated in FIG. 3C, couples firstcoil 108 and second coil 116 of the first channel in parallel to producea desired axial magnetic field polarization vector (parallel tolongitudinal axis 106 of former 100 as shown in FIG. 2). Pointconnectors 120 and 122 are electrically coupled as are point connectors124 and 126. First lead 148 of the transmit/receive circuit iselectrically coupled to point connectors 120 and 124 at the firsttransmit/receive terminal. Second lead 150 of the transmit/receivecircuit is electrically coupled to point connectors 120 and 126 at thesecond transmit/receive terminal. First coil 108 and second coil 116 arethus coupled in parallel with respect to transmit/receive circuit leads148 and 150. Again assuming that the construction of coils 108 and 116are essentially identical, a sinusoidal voltage applied acrosstransmit/receive leads 148 and 150 will produce a magnetic fieldparallel to longitudinal axis 106 of former 100.

A fourth configuration in which first 108 and second 116 coils of thefirst channel are connected in series to produce a desired axialmagnetic field polarization vector (parallel to longitudinal axis 106 offormer 100 as shown in FIG. 2) is shown in FIG. 3D. Point connectors 120and 126 are electrically coupled. First lead 148 of the transmit/receivecircuit is electrically coupled to point connector 122 at the firsttransmit/receive terminal. Second lead 150 of the transmit/receivecircuit is electrically coupled to point connector 124 at the secondtransmit/receive terminal. First coil 108 and second coil 116 are thuscoupled in series with respect to transmit/receive circuit leads 148 and150. A sinusoidal voltage applied across transmit/receive leads 148 and150, retaining the assumption of essentially identical construction ofcoils 108 and 116, will produce a magnetic field parallel tolongitudinal axis 106.

It will be apparent that connections at the first and secondtransmit/receive terminals may be reversed since a sinusoidal voltage isused.

As noted, these configurations are equally applicable to thecorresponding point connectors of the second channel coupling means.Each connector configuration for the second channel involves a first anda second transmit/receive terminal coupled to a third and a fourthtransmit/receive circuit lead, 152 and 154, respectively (FIGS. 5 and6).

By orienting the minor axes of the coils of the first channel A at a 90°angle with respect to the minor axes of the coils of the second channelB, the respective coupling means may be configured to obtain magneticfield polarization vectors of the respective first and second channelswhich are perpendicular to one another and to the static magnetic fieldB_(o). For example, use of the first connection configuration describedabove for both channels results in orthogonal radial magnetic fieldpolarization vectors. These orthogonal magnetic field vectors may beused to transmit and receive the X and Y components of the angular orrotational magnetic fields while the static magnetic field B_(o) is inthe Z direction parallel to longitudinal axis 106 of former 100.

An advantage of the present invention over conventional quadrature coilantenna designs is attributable to the minimal physical overlap of thecoils. The coils of a giver channel in the preferred embodiment overlapone another at a single point directly opposite the coupling means atthe end of the minor axes of the coils for coils having a single turn,and at only one additional point at the other end of the minor axes ofthe coils for coils having more than one turn. The coils of the firstchannel overlap the coils of the second channel at eight points, fourpoints on each side of reference plane 114.

Cross-talk between the coils of a given channel does not adverselyaffect performance of the MRI system since the two coils of that channelare electrically coupled at the coupling means and the same signal issimultaneously transmitted or received in both coils. However, asdescribed above with reference to conventional quadrature antennadesigns, cross-talk between the first and second channels does degradeperformance of the system since the assumption that noise in the systemis uncorrelated is no longer valid. Accordingly, the quadrature antennaof the preferred embodiment includes shielding means for shielding thefirst channel from the second channel.

The shielding means may include electromagnetic and electrostaticshielding. The shielding means of the preferred embodiment includeelectrostatic shielding means which limits or prevents a voltage in thecoil pair of one channel from being imparted to the coil pair of theother channel, thus electrically isolating or decoupling the firstchannel from the second channel. The shielding means of the preferredembodiment as illustrated in FIG. 4A comprises a plurality of shieldingplates 156 each including a grounded conductive sheet 158, such as ametallic foil or plate, which serves as an electromagnetic shieldsandwiched between first and second dielectric layers 160 and 162,respectively. Dielectric layers 160 and 162 prevent electricalconduction from the first channel A to the second channel B throughconductive sheets 158. Dielectric layers 160 and 162 may be of the sameor different composition. Preferred materials for the dielectric layersinclude polyethylene, polypropylene, and polystyrene. Also preferred arepolyfluoro hydrocarbons such as polytetrafluoroethylene.

As shown in FIG. 4B, the shielding means of the preferred embodimentcomprises a plurality of these sandwiched shielding plates 156, eachbeing a flat patch having a width approximately three times the width ofthe one of the respective coils which it shields having the greatestwidth. The flat surface of each of shielding plates 156 conforms to thecoils and cylinder wall 106 of former 100. An insulator 164 similar insize and shape to shielding plates 156 is also preferably positioned atthe coupling means and crossing points of each of the respective coilsof a given channel to prevent shorting of the coils at these point, asshown in FIG. 4B.

The first and second channels are 90° out of phase with respect to oneanother based on their physical location, as described above. Forexample, a magnetic field rotating inside former 100 with uniformangular velocity as produced by the precessing nuclei N and will producea sinusoidal signal on the first channel A that is coherent with and 90°out of phase with the signal on the second channel B. Conversely, asignal on one channel that is coherent with and 90° out of phase with asignal on the other channel can provide the rotating magnetic field B₁which can impart a corresponding rotational magnetic force on the objectwithin former 100. The transmit/receive circuit coupled to thequadrature coil antenna of the present invention is designed to takeadvantage of this phase angle relationship.

The quadrature antenna of the present invention can be used to transmitand receive RF signals. Each of the channels of the quadrature antennaof the present invention may be coupled to a transmit/receive circuitwhich provides a pulsed RF signal to the antenna and receives a responsesignal from the antenna, as described above. There is flexibility in theconfiguration of the transmit/receive circuit relative to the antenna.For example, both channels may be coupled to a single transmit/receivecircuit, or each channel may be coupled to a separate transmit/receivecircuit. Furthermore, the transmit/receive circuit may comprise aseparate transmitter and a separate receiver. To illustrate the couplingof the quadrature antenna of the present invention to a transmit/receivecircuit, and to illustrate the operation of the antenna in conjunctionwith the transmit/receive circuit, the transmit/receive circuit will beassumed to include a separate transmitter and a separate receiver. Twoembodiments of a quadrature transmitter and two embodiments of aquadrature receiver will be described. It is emphasized that thetransmit/receive circuits discussed here are by way of example and notlimitation.

The first quadrature transmitter configuration 166, shown in FIG. 5A,comprises an amplifier 168 coupled to a phase shifter 170, the latter ofwhich preferably comprises a quadrature hybrid network. Phase shifter170 includes a signal splitter and first and second channels. The firstchannel has a first and a second output port 172 and 174, respectively.First output port 172 is electrically coupled to the firsttransmit/receive terminal 176 of the first channel A of the antenna byfirst transmit/receive circuit lead 148. Second output port 174 iselectrically coupled to the second transmit/receive terminal 178 of thefirst antenna channel A by second transmit/receive circuit lead 150. Thesecond channel of phase shifter 170 also has a first and a second outputport 180 and 182, respectively. First output port 180 is electricallycoupled to the first transmit/receive terminal 184 of the second channelB of the antenna by third transmit/receive circuit lead 152. Secondoutput port 182 is electrically coupled to the second transmit/receiveterminal 186 by fourth transmit/ receive circuit lead 154. Tuningcapacitors 188 and 190 are placed between the first and secondtransmit/receive terminals of the respective channels of the antenna.

The operation of the quadrature transmitter configuration of FIG. 5A isas follows. An RF signal is provided at the input terminals of amplifier168 where the signal is amplified and transmitted to phase shifter 170.The signal splitter splits the signal into two identical signals--afirst channel signal and a second channel signal. The first channelsignal is transmitted to first output port 172 and second output port174 of the first channel of phase shifter 170. The first channel signalis then transmitted to the first channel A of the antenna by first andsecond transmit/receive circuit leads 148 and 150. The phase of thesecond channel signal is shifted by 90° with respect to the phase of thefirst channel signal by phase shifter 170. This phase shifted secondchannel signal is then provided at first output port 180 and secondoutput port 182 of phase shifter 170 from which it is transmitted to thesecond channel B of the antenna by transmit/receive circuit leads 152and 154, while retaining its phase relationship with respect to thefirst channel signal. Thus, this quadrature transmitter converts asingle RF signal into two coherent signals having a 90° phasedifference. These signals are provided to the respective channels of theantenna of the present invention which transmits these signals along twoseparate perpendicular axes (X and Y) to provide the circularlypolarized magnetic field B₁ which causes precession.

A second quadrature transmitter configuration 192 is shown in FIG. 5B.This configuration differs from the configuration 166 shown in FIG. 5Ain that a separate amplifier is used for each channel. The quadraturetransmitter configuration 192 shown in FIG. 5B comprises a phase shifter194, such as a quadrature hybrid circuit, coupled to a first channelamplifier 196 and a second channel amplifier 198. Phase shifter 194comprises a signal splitter and a first and a second channel. Each ofthe first and second channels has an output port. The output port 200 ofthe first channel is coupled to the input terminal 202 of first channelamplifier 196. The output port 204 of the second channel is coupled tothe input terminal 206 of second channel amplifier 198. First channelamplifier 196 has a first output terminal 208 coupled to firsttransmit/receive terminal 176 of the first channel A of the antenna byfirst transmit/receive circuit lead 148. A second output terminal 210 offirst channel amplifier 196 is coupled to second transmit/ receiveterminal 178 of the first channel A of the antenna by secondtransmit/receive circuit lead 150. Similarly, second channel amplifier198 has a first output terminal 212 coupled to first transmit/receiveterminal 184 of the second channel B of the antenna by thirdtransmit/receive circuit lead 152, and a second output terminal 214coupled to the second transmit/receive terminal 186 of the secondchannel B of the antenna by fourth transmit/receive circuit lead 154.Tuning capacitors 216 and 218 are placed between the first and secondtransmit/receive terminals of the respective channels of the antenna.

The operation of the quadrature transmitter configuration 192 of FIG. 5Bis as follows. A single RF signal is provided at the input to phaseshifter 194. The signal splitter splits the signal into two coherentsignals, again a first and a second channel signal. The first channelsignal is transmitted to output port 200 of the first channel of phaseshifter 194 and to the input terminal 202 of the first channel amplifier196. First channel amplifier 196 amplifies this first channel signal andapplies it across its output terminals 208 and 210, thus providing theamplified first channel signal to transmit/receive terminals 176 and 178of the first channel A of the antenna by transmit/receive circuit leads148 and 150, respectively. The phase of the second channel signal isshifted by 90° with respect to the phase of the first channel signal byphase shifter 194. This phase shifted second channel signal is thenprovided to output port 204 of the second channel of phase shifter 194from which it is transmitted to the input terminal 206 of second channelamplifier 198. Second channel amplifier 198 amplifies this phase shiftedsecond channel signal and applies it across its output terminals 212 and214 while retaining the phase relationship of the first and secondchannel signals. In this manner, the second channel signal is providedto transmit/receive terminals 184 and 186 of the second channel B of theantenna by transmit/receive circuit leads 152 and 154, respectively.Configuration 192 results in essentially the same result as transmitterconfiguration 166.

Two quadrature receiver configurations are shown in FIG. 6. The firstquadrature receiver configuration 220, shown in FIG. 6A, is analogous toquadrature transmitter configuration 166 of FIG. 5A. It includes phaseshifter 222 coupled to output amplifier 224. Phase shifter 222, whichmay be a quadrature hybrid network, includes a quadrature combiner and afirst and second channel. Each of the first and second channels has twoinput ports. Phase shifter 222 includes input ports 226 and 228 of thefirst channel coupled to transmit/receive terminals 176 and 178 of thefirst channel A of the antenna by transmit/receive circuit leads 148 and150, respectively. Phase shifter 222 also includes input ports 230 and232 of the second channel coupled to transmit/receive terminals 184 and186 of the second channel B of the antenna by transmit/receive circuitleads 152 and 154, respectively. The output of phase shifter 222 iscoupled to output amplifier 224. Tuning capacitors 234 and 236 areplaced between the first and second transmit/receive terminals of therespective channels of the antenna.

A rotating RF signal generated by the precessing nuclei N of an objectwithin the antenna is applied at transmit/receive terminals 176 and 178of the first channel A to create a first channel signal. Ninety degrees(90°) later as the magnetization rotates, this signal is applied atfirst and second transmit/receive terminals 184 and 186 of the secondantenna channel B to create a second channel signal 90° out of phasewith respect to the first channel signal. The first channel signal istransmitted to input ports 226 and 228 of the first channel of phaseshifter 222 and the second channel signal is transmitted to input ports230 and 232 of the second channel of phase shifter 232. The phase of thesecond channel signal is shifted by 90° by phase shifter 222 so that itis in phase with the first channel signal. The first channel signal isthen combined with the phase shifted second channel signal by thequadrature combiner. The combined signal is transmitted to outputamplifier 224. Thus, the antenna receives both components (X and Y) ofthe RF signal generated by the precessing nuclei N. Although the signalis received on two separate channels A and B of the antenna, thequadrature combiner combines the signals of the respective channels inphase so that the enhanced sensitivity noted above is obtained.

Quadrature receiver configuration 238 of FIG. 6B is analogous toquadrature transmitter configuration 192 shown in FIG. 5B, and differsfrom the first receiver configuration 220 shown in FIG. 6A in that thefirst and second channel signals are independently amplified prior toentering the phase shifter rather than after being combined.Transmit/receive terminals 176 and 178 of the first channel A of theantenna are coupled to the respective input terminals 240 and 242 of afirst channel amplifier 244 by transmit/receive terminal leads 148 and150. Transmit/receive terminals 184 and 186 of the second channel B ofthe antenna are coupled to the respective input terminals 246 and 248 ofa second channel amplifier 250. The output terminals 252 and 254 of thefirst and second channel amplifiers are coupled to the input ports 256and 258 of the first and second channels, respectively, of phase shifter260. Phase shifter 260 comprises a quadrature combiner and first andsecond channels. Tuning capacitors 262 and 264 are placed between thefirst and second transmit/receive terminals of the respective channelsof the antenna.

The circularly polarized rotating magnetic field described above isapplied to the antenna by the precessing nuclei N, which results in afirst channel signal on the first channel A of the antenna and a secondchannel signal shifted in phase by 90° with respect to the first channelsignal on the second channel B of the antenna. Each of these signals istransmitted to the respective amplifiers 244 and 250 where they areamplified and transmitted to the respective input ports 256 and 258 ofphase shifter 260. The phase of the second channel signal is shifted by90° by phase shifter 260 so that it is in phase with the first channelsignal. The first channel signal is then combined with the phase-shiftedsecond channel signal by the quadrature combiner. The combined signal isthe output of phase shifter 260. Thus, essentially the same result asfor quadrature receiver configuration 220 is obtained.

Additional advantages and modifications will readily occur to thoseskilled in the art. For example, the coils of the quadrature antenna maybe mounted with respect to one another as described herein without theuse of a former. In this case, the longitudinal axis of the antenna isthe axis which runs through the center point of the coils and isinclined approximately 45° with respect to each of the first throughfourth planes. Equivalently, the longitudinal axis of the antenna isperpendicular to the plane formed by the minor axes of the first throughfourth coils (i.e., reference plane 114). Thus, the longitudinal axis ofthe antenna is the axis which would have been longitudinal axis 106 offormer 100 had a former been used.

As an additional example, the construction of the coils may vary fromone coil to another, e.g., the number of windings, the size of the coil,and the dimensions and electrical conductivity of the conductor.Appropriate adjustments would be required to ensure that the magneticfields produced by the coils were substantially orthogonal and ofroughly equal or known and accomodated magnitude. Also, adjustmentswould be required for any phase variations introduced into the first orsecond channels.

The invention in its broader aspects is, therefore, not limited to thespecific details, representative apparatus and illustrative examplesshown and described. Accordingly, departures may be made from suchdetails without departing from the spirit or scope of the generalinventive concept as defined by the appended claims and theirequivalents.

What is claimed is:
 1. A quadrature antenna for magnetic resonanceimaging having a longitudinal axis, said antenna comprising:a firstchannel including,a first elliptical coil lying in a first plane, thecenter point of said first coil coinciding with a reference point onsaid longitudinal axis, the minor axis of said first coil lying in areference plane, said reference plane intersecting said longitudinalaxis at said reference point and being normal to said longitudinal axis,said first plane being inclined approximately forty five degrees (45°)with respect to said longitudinal axis, a second elliptical coil lyingin a second plane, the center point of said second coil coinciding withsaid reference point, the minor axis of said second coil lying in saidreference plane and being colinear with the minor axis of said firstcoil, said second plane being normal to said first plane and inclinedapproximately forty five degrees (45°) with respect to said longitudinalaxis, and means for electrically coupling said first channel to anexternal transmit/receive circuit; and a second channel including, athird elliptical coil lying in a third plane, the center point of saidthird coil coinciding with said reference point, the minor axis of saidthird coil lying in said reference plane and being perpendicular to theminor axis of said first coil, said third plane being inclinedapproximately forty five degrees (45°) with respect to said longitudinalaxis, a fourth elliptical coil lying in a fourth plane, the center pointof said fourth coil coinciding with said reference point, the minor axisof said fourth coil lying in said reference plane and being colinearwith the minor axis of said third coil, said fourth plane being normalto said third plane and inclined approximately forty five degrees (45°)with respect to said longitudinal axis, and means for electricallycoupling said second channel to an external transmit/receive circuit. 2.An antenna as recited in claim 1, wherein said first coil and saidsecond coil are electrically coupled in series.
 3. An antenna as recitedin claim 1, wherein said first coil and said second coil areelectrically coupled in parallel.
 4. An antenna as recited in claim 1,wherein said third coil and said fourth coil are electrically coupled inseries.
 5. An antenna as recited in claim 1, wherein said third coil andsaid fourth coil are electrically coupled in parallel.
 6. An antenna asrecited in claim 1, further comprising means for shielding said firstchannel from said second channel.
 7. An antenna as recited in claim 6,wherein said shielding means comprises means for electrostaticallyshielding said first channel from said second channel, saidelectrostatic shielding means including a first dielectric layer, asecond dielectric layer, and a grounded conductive layer sandwichedbetween said first dielectric layer and said second dielectric layer. 8.An antenna as recited in claim 7, wherein at least one of said first andsecond dielectric layers comprises at least one of polyethylene,polypropylene, polystyrene, and a polyfluoro hydrocarbon.
 9. An antennaas recited in claim 7, wherein at least one of said dielectric layerscomprises polytetrafluoroethylene.
 10. An antenna as recited in claim 1,further comprising a cylindrical former having a cylinder wall and alongitudinal axis, said first, second, third and fourth coils beingwound to conform to said cylinder wall, and said longitudinal axis ofsaid former being substantially colinear with said longitudinal axis ofsaid antenna.
 11. An antenna as recited in claim 1, further comprisingan elliptic cylinder former having a cylinder wall and a longitudinalaxis, said first, second, third and fourth coils being wound to conformto said cylinder wall, and said longitudinal axis of said former beingsubstantially colinear with said longitudinal axis of said antenna. 12.A quadrature antenna for magnetic resonance imaging having alongitudinal axis, said antenna comprising:a first channel including,afirst coil comprising at least one winding of an elongated conductor, acentral one of said first coil windings lying substantially in a firstplane and forming a first ellipse, the other of said first coil windingsbeing adjacent and having substantially equal dimensions to said firstcoil central winding and being parallel to said first plane, the centerpoint of said first ellipse coinciding with a reference point on saidlongitudinal axis, the minor axis of said first ellipse lying in areference plane, said reference plane intersecting said longitudinalaxis at said reference point and being normal to said longitudinal axis,said first plane being inclined approximately forty five degrees (45°)with respect to said longitudinal axis, a second coil comprising atleast one winding of an elongated conductor, a central one of saidsecond coil windings lying substantially in a second plane and forming asecond ellipse, the other of said second coil windings being adjacentand having substantially equal dimensions to said second coil centralwinding and being parallel to said second plane, the center point ofsaid second ellipse coinciding with said reference point, the minor axisof said second coil lying in said reference plane and being colinearwith the minor axis of said first ellipse, said second plane beinginclined approximately forty five degrees (45°) with respect to saidlongitudinal axis, and first channel coupling means for electricallycoupling said first channel to an external transmit/receive circuit; anda second channel including,a third coil comprising at least one windingof an elongated conductor, a central one of said third coil windingslying substantially in a third plane and forming a third ellipse, theother of said third coil windings being adjacent and havingsubstantially equal dimensions to said third coil central winding andbeing parallel to said third plane, the center point of said thirdellipse coinciding with said reference point, the minor axis of saidthird ellipse lying in said reference plane and being perpendicular tothe minor axis of said first ellipse, a fourth coil comprising at leastone winding of an elongated conductor, a central one of said fourth coilwindings lying substantially in a fourth plane and forming a fourthellipse, the other of said fourth coil windings being adjacent andhaving substantially equal dimensions to said fourth coil centralwinding and being parallel to said fourth plane, the center point ofsaid fourth ellipse coinciding with said reference point, the minor axisof said fourth ellipse lying in said reference plane and being colinearwith the minor axis of said third ellipse, and second channel couplingmeans for electrically coupling said second channel to an externaltransmit/receive circuit.
 13. An antenna as recited in claim 12, whereinsaid first coil and said second coil are electrically coupled in series.14. An antenna as recited in claim 12, wherein said first coil and saidsecond coil are electrically coupled in parallel.
 15. An antenna asrecited in claim 12, wherein said third coil and said fourth coil areelectrically coupled in series.
 16. An antenna as recited in claim 12,wherein said third coil and said fourth coil are electrically coupled inparallel.
 17. An antenna as recited in claim 12, further comprisingshielding means for shielding said first channel from said secondchannel.
 18. An antenna as recited in claim 16, wherein said shieldingmeans comprises means for electrostatically shielding said first channelfrom said second channel, said electrostatic shielding means including afirst dielectric layer, a second dielectric layer, and a groundedconductive layer sandwiched between said first dielectric layer and saidsecond dielectric layer.
 19. An antenna as recited in claim 18, whereinat least one of said first and second dielectric layers comprises atleast one of polyethylene, polypropylene, polystyrene, and a polyfluorohydrocarbon.
 20. An antenna as recited in claim 18, wherein at least oneof said dielectric layers comprises polytetrafluoroethylene.
 21. Anantenna as recited in claim 12, further comprising a cylindrical formerhaving a cylinder wall and a longitudinal axis, said first, second,third and fourth coils being wound to conform to said cylinder wall, andsaid longitudinal axis of said former being substantially colinear withsaid longitudinal axis of said antenna.
 22. An antenna as recited inclaim 12, further comprising an elliptic cylinder former having acylinder wall and a longitudinal axis, said first, second, third andfourth coils being wound to conform to said cylinder wall, and saidlongitudinal axis of said former being substantially colinear with saidlongitudinal axis of said antenna.